This former question is about the mobility constant $M>0$ in the Cahn-Hilliard equation. To determine the value of such a mobility for a simulation study, Kim and Sanders (2020) use the formula $$ M = \frac{D \cdot N_A}{\partial^2 G / \partial c^2},\qquad \text{(*)} $$ where $D$ is the diffusion coefficient, $N_A$ is the number of particles per unit area, $G$ is the molar Gibbs free energy and $c$ is the molar fraction.
Question: Where does the formula (*) come from? Does this formula have something do to with the Einstein relation?
